The term "thermal lamination" with a film in the present invention means to laminate the film on a substrate such as a metal or a non-metal (paper, plastics, fibers, non-woven fabric or the like) wherein the substrate or the film is heated to continuously adhere the both to form a composite.
Particularly, methods for laminating a film on a metal plate are drawing attention with the recent increase in the production of metal cans.
Conventionally, in order to prevent corrosion of inner surface of metal cans, it is widely performed to apply solutions or dispersions of various thermosetting resins such as epoxy resins and phenol resins in a solvent to the inner surface of metal cans to coat the inner surface. However, with this method by which the inner surface is coated with a thermosetting resin, a long time is required for drying the coating liquid, so that the production efficiency is low. Further, environmental pollution by the large amount of organic solvents is problematic.
To overcome these problems, a polyester film is laminated on a steel plate, aluminum plate or on a steel or aluminum plate whose surface is treated in wide variety of manners such as plating, and a metal can is prepared from the metal plate laminated with the film by drawing or ironing. The following characteristics are demanded for the polyester film:
(1) The film has a good adhesion to a metal plate. PA0 (2) The film has a good shapability and voids such as pin holes are not formed after being shaped. PA0 (3) The polyester film is not peeled off and cracks and pin holes are not formed in the film by an impact to the metal can. PA0 (4) Smelling components of the content in the can are not adsorbed by the polyester film, and the taste of the content is not deteriorated by a component eluted from the polyester film (This property is hereinafter referred to as "taste characteristics").
To satisfy these demands, a number of proposals have been made. For example, Japanese Laid-open Patent Application (Kokai) No. 2-305827 discloses a polyester film having a specific coefficient of planar orientation, and Japanese Laid-open Patent Application (Kokai) No. 2-57339 discloses a copolymerized polyester film having a specific crystallinity. However, by these proposals, the above-mentioned wide variety of demands cannot be simultaneously satisfied. Particularly, it is difficult to simultaneously attain good shapability and good impact resistance, and the taste characteristics are unsatisfactory. Especially, in the ironing, since the film is drawn at a ratio of about 200-300%, it is difficult to attain good characteristics with the above-described conventional films.
Further, when the film is thermally laminated with a substrate other than metal, it is difficult to obtain a composite which satisfies good adhesion, impact resistance and taste characteristics. In particular, with the container having paper and on olefin-based polymer laminated on the paper, the taste characteristics are extremely bad.